They are harder to setup than other designs that do the same thing. If you want the axle to travel directly up and down it is easier to fabricate and A frame or triangulated uppers which control the lateral location of the axle as well as the rotation of the axle. A watts linkage is more complex and fiddly for a fabricator in comparison and only provides lateral location.
You will also compromise lateral stability for road use. No road going vehicle that I'm aware of runs that setup. If you do, then Rose joints in the top triangulation would be my recommendation.
I don't know what he is running but a Watts linkage will do that.

